Synthetic quartz glass optical material and optical member for f2 excimer lasers

ABSTRACT

An object of the present invention is to provide a synthetic quartz glass optical material having a high optical transmittance for a radiation 157 nm in wavelength emitted from F 2  excimer laser and a high resistance against irradiation of a F 2  excimer laser radiation, yet having a uniformity suitable for such a fine patterning using a F 2  excimer laser, and to provide an optical member using the same.  
     The problems above are solved by a synthetic quartz glass optical material for F 2  excimer lasers having an OH group concentration of 0.5 ppm or lower, a fluorine concentration of 0.1 to 2 mol %, a hydrogen molecule concentration of 5×10 16  molecules/cm 3  or lower, a difference between the maximum and minimum fluorine concentrations within 20 mol ppm, and a difference between the maximum and minimum refraction indices of 2×10 −5  or lower.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a synthetic quartz glass opticalmember that transmits a F₂ excimer laser radiation such as a lens, aprism, a filter, a photomask, etc., and to an optical material for thesame.

PRIOR ART

[0002] The technique of photolithography, which transfers a circuitpattern on a mask to a wafer by using light, is economically superior toother techniques using electron beam or an X-ray radiation, and isthereby widely used as an exposure technique for producing integratedcircuits.

[0003] With the recent demand for further miniaturized and highlyintegrated LSIs, radiations with shorter wavelengths are being used asthe light source, and aligners using i-lines 365 nm in wavelengthcapable of forming patterns with line widths in a range of from 0.5 to0.4 μm or those using KrF excimer laser radiations capable of formingpatterns with line widths in a range of from 0.18 to 0.35 μm have beenput to practical use. Moreover, an aligner using an ArF excimer laseremitting radiations 193 nm in wavelength and capable of forming patternswith line widths in a range of from 0.10 to 0.20 μm is now placed underdevelopment.

[0004] As an apparatus for the next age lithography, there are studiedan electron beam direct patterning technology, a X-ray proximity ionlithography, and a F₂ excimer laser exposure technology. Among them,electron beam direct writing technology has a problem concerning itsthroughput, and the X-ray proximity ion lithography suffers a problem informing the mask. Thus, the F₂ excimer laser exposure technology isattracting most attention as the next age exposure technology.

[0005] From the view point of transmittance, resistance against laserradiations, uniformity, etc., a quartz glass, and particularly, a highpurity synthetic quartz glass has been used as the optical material foruse in the conventional excimer lasers using KrF, ArF, etc. A quartzglass exhibits high optical transmittance in the wavelength region forthe KrF and ArF laser radiations, and the resistance against laserradiation can be increased by optimizing the conditions of production.Hence, an optical material, and particularly such usable as a projectionlens, is already available.

[0006] However, since a F₂ excimer laser generates a radiation at awavelength of 157 nm, i.e., a wavelength further shorter than that of anArF excimer laser, it was found impossible to obtain a sufficiently hightransmittance with the synthetic quartz glass used conventionally for aKrF or an ArF excimer laser. Hence, the optical material usable for theF₂ excimer laser was limited only to fluorite. This was a greatlimitation in designing the apparatus.

[0007] On the other hand, in JP-A-Hei4-195101 (the term “JP-A-” asreferred herein signifies “an unexamined published Japanese patentapplication”) is disclosed a technology for reducing or extinguishingthe defects that absorb the radiation in the wavelength region of from155 to 400 nm, and for preventing the generation of defects even in casea high energy ultraviolet radiation is irradiated for a long duration oftime. However, the effect of the technology concerning the resistanceagainst a laser radiation is confirmed only for an ArF excimer laserradiation, and there is no description concerning the resistance againsta radiation of a F₂ excimer laser.

[0008] Furthermore, JP-A-Hei8-67530 discloses a technology forincreasing the stability against an ArF excimer laser radiation bydoping a quartz glass with 1% by weight or more of fluorine and 10 ppmor more of OH groups. This publication also shows a considerableimprovement in the transmittance for ultraviolet radiation in thevicinity of 157 nm, i.e., in the wavelength region of a F₂ excimer laserradiation.

[0009] Problems the Invention is to Solve

[0010] Surely, the optical stability of a quartz glass against an ArFexcimer radiation can be improved by incorporating fluorine into thequartz glass. However, in case of a F₂ excimer laser radiation with afurther shorter wavelength as compared with the ArF excimer laserradiation, it has been found that the technology above is not capable ofsufficiently suppressing the generation of defects attributed to theirradiation of an excimer laser radiation.

[0011] The present inventors studied the physical properties and thedamage behavior of a quartz glass under the irradiation of a F₂ excimerlaser radiation in case the ultraviolet radiation is changed to that ofa F₂ excimer laser. As a result, the glass characteristics suitable fora quartz glass material for optical use together with a F₂ excimer laserhas been discovered. The present invention has been accomplished basedon these findings.

[0012] More specifically, an object of the present invention is toprovide a synthetic quartz glass optical material having a high opticaltransmittance for a radiation 157 nm in wavelength emitted from F₂excimer laser and a high resistance against irradiation of a F₂ excimerlaser radiation, yet having a uniformity suitable for such a finepatterning using a F₂ excimer laser, and to provide an optical memberusing the same.

[0013] Means for Solving the Problems

[0014] The problems above are solved by any of the constitutions (1) to(8) described below.

[0015] (1) A synthetic quartz glass optical material for F₂ excimerlasers having an OH group concentration of 0.5 ppm or lower, a fluorineconcentration of 0.1 to 2 mol %, a hydrogen molecule density of 5×10¹⁶molecules/cm³ or lower, a difference between the maximum and minimumfluorine concentrations within 20 mol ppm, and a difference between themaximum and minimum refraction indices of 2×10⁻⁵ or lower.

[0016] (2) A synthetic quartz glass optical material for F₂ excimerlasers having an OH group concentration of 0.5 ppm or lower, a fluorineconcentration of 0.1 to 2 mol %, a hydrogen molecule density of 5×10¹⁶molecules/cm³ or lower, a difference between the maximum and minimumfluorine concentrations within 100 mol ppm, a difference between themaximum and minimum fictive temperatures within 50° C., and a differencebetween the maximum and minimum refraction indices being set to 2×10⁻⁵or lower by relatively forming a fluctuation in refractive indices inaccordance with the fictive temperature, in such a manner that thefluctuation in refractive indices attributed to the distribution in theconcentration of fluorine be cancelled.

[0017] (3) A synthetic quartz glass optical material for F₂ excimerlasers as described in constitution (2) above, wherein the maximum valuein the distribution of fictive temperature is 920° C. or lower.

[0018] (4) A synthetic quartz glass optical material for F₂ excimerlasers as described in one of constitutions (1) to (3) above, whereinthe internal transmittance for a radiation 157 nm in wavelength emittedfrom F₂ excimer lasers is 70% or higher.

[0019] (5) A synthetic quartz glass optical material for F₂ excimerlasers as described in one of constitutions (1) to (3) above, whereinthe internal transmittance for a radiation 163 nm in wavelength is 90%or higher.

[0020] (6) A synthetic quartz glass optical material for F₂ excimerlasers as described in one of constitutions (1) to (3) above, whereinthe drop in transmittance for a radiation 157 nm in wavelength afterirradiating thereto 3×10⁶ pulses of F₂ excimer laser radiation at anenergy density per pulse of 10 mJ/cm² is 5% per 10 mm or less.

[0021] (7) A synthetic quartz glass optical material for F₂ excimerlasers as described in one of constitutions (1) to (3) above, whereinthe birefringence measured at a wavelength of 633 nm is 0.5 nm/cm orlower.

[0022] (8) A synthetic quartz glass optical member for F₂ excimer lasersformed by using a synthetic quartz glass optical material as describedin one of constitutions (1) to (7) above.

[0023] Embodiments for Practicing the Invention

[0024] It is required that the synthetic quartz glass according to thepresent invention is a synthetic quartz glass obtained by a so-calledsoot process. A synthetic quartz glass obtained by the soot process canachieve a combination of requirements necessary for solving theproblems, i.e., a synthetic quartz glass having a low OH groupconcentration and a high fluorine concentration.

[0025] The synthetic quartz glass is obtained from a soot processbriefly described below, comprising a step of forming a porous silicamother body (which is referred to hereinafter as “a soot body”), inwhich a volatile silicon raw material, for instance, a silicon halidematerial represented by SiCI₄, an alkoxysilane such as SiCH₃(OCH₃),Si(OCH₃), etc., or a siloxane such as Si(CH₃)₃OSi(CH₃)₃, (Si(CH₃)₃O₄,etc., is hydrolyzed in a combustion flame containing oxygen andhydrogen, and the fine silica particles generated thereby are depositedon a rotating base body; a step of doping the resulting soot body withfluorine by thermally treating the soot body under an atmospherecontaining fluorine; and a step of vitrifying the resulting soot body toobtain a transparent glass body.

[0026] The synthetic quartz glass optical material and an optical memberfor use with a F₂ excimer laser according to the present invention arecharacterized by that they have an OH group concentration of 0.5 ppm orlower, a fluorine concentration of 0.1 to 2 mol %, a hydrogen moleculedensity of 5×10¹⁶ molecules/cm³ or lower, and a difference between themaximum and minimum fluorine concentrations within 20 mol ppm. In thismanner, the synthetic quartz glass optical material and an opticalmember according to the present invention achieves an improveduniformity capable of yielding a difference between the maximum andminimum refraction indices of 2×10⁻⁵ or lower, while improving theresistance against laser irradiation as such capable of minimizing thedrop in transmittance from the high initial value even after anirradiating a F₂ excimer laser radiation for a prolonged duration oftime.

[0027] In the synthetic quartz glass optical material according to thepresent invention, the OH group concentration is confined to be 0.5 ppmor lower based on the reasons as follows.

[0028] Conventionally, OH groups were believed to increase stabilityagainst the irradiation of F₂ excimer laser radiations. However, sinceOH groups absorb radiations with energy of 7.8 eV (159 nm), they notonly reduces the transmittance of F₂ excimer laser radiation with awavelength of 159 nm, but also severely influence the stability againstthe irradiation of a F₂ excimer laser radiation.

[0029] That is, it has been found that the irradiation of a F₂ excimerradiation generates NBOHCs (Non-Bridged Oxygen Hole Centers), and thatit thereby is the cause of generating defects.

[0030] In the light of such circumstances, the upper limit of the OHgroup concentration was set, as described above, to 0.5 ppm or lower,preferably, to 0.1 ppm or lower. In fact, on irradiating a F₂ excimerlaser radiation to a quartz glass containing OH groups at aconcentration of about 10 ppm, it has been confirmed that E′ (E prime)centers having an absorption peak at 215 nm generate together with anintense NBOHC having an absorption peak at a wavelength of 260 nm. Atthe same time, the infrared absorption peak at a wavenumber of 3680 cm⁻¹assigned to OH groups was found to be shifted to the lower wavenumberside by about 26 cm⁻¹, while lowering its peak intensity by about 6%.

[0031] The above phenomenon was not observed in the conventional casesof irradiating KrF or ArF excimer laser radiations, and it is believedattributed to the generation of NBOHCs, resulting from the irradiationof an intense ultraviolet radiation by a F₂ excimer laser, whichaffected the O-H bonds in Si-OH groups. Accordingly, the OH groupconcentration must be confined to 0.5 ppm or lower.

[0032] Next, the reason for setting the fluorine concentration in arange of from 0.1 to 2% by mole in the synthetic quartz glass opticalmaterial according to the present invention is described below.

[0033] In the synthetic quartz glass optical material according to thepresent invention, the fluorine concentration is confined in the rangedescribed above. In this manner, the induction of paramagnetic defects,i.e., the E′ centers (E prime centers), is prevented from occurring onirradiating a F₂ excimer laser radiation. Thus, the drop in opticaltransmittance of a radiation 157 nm in wavelength attributed to thegeneration of E′ centers is suppressed to thereby suppress the decreasein resistance against irradiation of F₂ excimer laser radiations.

[0034] If fluorine should be incorporated at a concentration lower than0.1% by mole, the effects above cannot be sufficiently obtained; on theother hand, if fluorine should be doped into the quartz glass at anamount exceeding 2% by mole, the quartz glass tends to induce oxygendeficient defects. Thus, fluorine is preferably incorporated in thequartz glass in a range of from 0.1 to 2% by mole, and more preferably,in a range of from 0.7 to 1.5% by mole.

[0035] In the present invention, the fluorine concentration in thequartz glass is realized by doping F. As the F source, preferably usedare, for instance, gaseous SiF₄ or SF₆. Gaseous SiF₄ or gaseous SF₆provides a gaseous atmosphere having a dehydration ability whilesuppressing the formation of oxygen deficient defects, and achieves aquartz glass having a low OH group concentration and a low oxygendeficient defects. Among the gaseous atmosphere above, SiF₄ is preferredin view of its ability for suppressing the generation of oxygendeficiency. As other gases having a dehydrating ability, there can bementioned gaseous chlorine, i.e., a gaseous halogen similar to thoseenumerated above; however, gaseous chlorine is not preferred because itis apt to generate oxygen deficiencies.

[0036] Next, the reason for setting the density of hydrogen molecules ina range of 5×10¹⁶ molecules/cm³ or lower is described below. When a F₂excimer laser radiation is irradiated to a quartz glass containinghydrogen molecules at a density of about 1×10¹⁷ molecules/cm³, thetransmittance for a radiation 157 nm in wavelength was found to decreaseeven though the concentrations for fluorine and OH groups were set totheir desired ranges. In the resulting quartz glass, the generation ofOH groups was observed after the irradiation of the laser radiation, andat the same time, NBOHCs, which are believed to be induced from the OHgroups, were observed to form inside the quartz glass.

[0037] It has been found that defects similar to those that form insidethe quartz glass containing OH groups on irradiating a laser radiationgenerate because the hydrogen molecules that are incorporated dissolvedinside the quartz glass generate OH groups by forming bonds with theoxygen present inside the quartz glass. If the density of hydrogenmolecules should exceed the range defined above, the resistance againstlaser radiation becomes degraded, and the transmittance for radiation157 nm in wavelength thereby decreases on irradiating a F₂ excimer laserradiation.

[0038] What is important in the requirements on the synthetic quartzglass optical material required in the present invention is that thehigh transmittance is maintained to an extremely high wavelength regionof 157 nm corresponding to that of the extreme ultraviolet wavelengthregion (for instance, preferred is that the internal transmittance of70% or higher, particularly preferably 80% or higher, and furtherpreferably, 90% or higher, is maintained for the wavelength of 157 nm ofthe F₂ excimer laser radiation). In order to achieve the requirements,the concentration of metallic impurities that shifts the absorption edgeto the higher wavelength side must be lowered as much as possible, andmust be controlled in the ppb level. Accordingly, a volatile siliconcompound with extremely high purity must be selected as the rawmaterial. Furthermore, particular consideration must be made to suppressthe contamination due to impurities during the entire process.

[0039] To achieve such a high transmittance as described above, thetotal concentration of five transition metals, i.e., Cu, Ni, Ti, Cr, andFe, should be controlled to 30 ppb or lower, that of three alkali metalelements, i.e., Na, Li, and K, should be controlled to 50 ppb or lower,and that of three alkaline earth metal elements, i.e., Ca, Ba, and Mg,should be controlled to 80 ppb or lower.

[0040] Furthermore, the transmittance in the wavelength region above isgreatly influenced by the dissolved gas. If gaseous oxygen and gaseousozone should be present inside the quartz glass optical material, theycause absorption of radiations in the ultraviolet region. This shiftsthe absorption edge to the longer wavelength side and, at the same time,this becomes a key cause of generating defects such as the NBOHCs. Thus,the concentration of dissolved gas is preferably as low as possible.

[0041] Accordingly, the transmittance for a radiation 157 nm inwavelength itself functions as an important measure representing thestability with respect to the laser radiation.

[0042] Furthermore, the quartz glass optical material according to thepresent invention preferably yields an internal transmittance of 90% orhigher for a radiation 163 nm in wavelength.

[0043] In order to assure a practical stability in case of using quartzglass as an optical material, preferably, the quartz glass opticalmaterial for use in transmitting F₂ excimer laser radiation according tothe present invention yields a drop in transmittance for a radiation 157nm in wavelength confined to 5% or less per 10 mm after irradiating F₂excimer laser radiation in pulses for 3×10⁵ repetition times with anenergy density of 10 mJ/cm².

[0044] The above condition corresponds to a drop in transmittance for apractical case in which the transmission energy is 0.1 mJ/cm2 for arange of 3×10⁷ to 3×10⁹ pulses, and assures a durability sufficientlyhigh as an exchangeable optical component.

[0045] Furthermore, the synthetic quartz glass optical materialaccording to the present invention preferably yields a birefringence of0.5 nm/cm or lower in case of measurement using a radiation 633 nm inwavelength.

[0046] A precise measurement for the birefringence can be carried out byusing an ellipsometer equipped with a He/Ne laser (emitting light at awavelength of 633 nm), and by measuring the difference in optical path(i.e., retardation, Δnd) between two polarized lights that are generatedby the birefringence, each having their direction of vibration crossingeach other at the right angle. For instance, in a specimen 5 cm inthickness and having an observed retardation of 20 nm, the birefringencecan be obtained as 4 nm/cm by dividing the retardation by the thickness.

[0047] The influence of birefringence of a practically employed opticalmember is determined by evaluating the retardation with respect thewavelength. For instance, in the example above with a retardation of 20nm, the evaluation can be made by dividing 20 nm by 633 nm, i.e.,20/633, and this equals to 0.0316λ (i.e., <λ/30). In this case, thematerial can be used without any problem, however, in case a light 153nm in wavelength is used, the same retardation is evaluated as20/157=0.127λ (i.e., aboutλ/8). Accordingly, the birefringence in thelatter case is more than four times as large as that of the former case.Thus, there remains possibility of causing problems. Furthermore, sincethe wavelength region corresponds to such in which the photoelasticconstant exhibits wavelength dependence, the birefringence that ismeasured at the wavelength of 633 nm becomes larger for a radiation 157nm in wavelength. In this context, it is important that the opticalmaterial for use with F₂ excimer lasers should yield a birefringence ata wavelength of 633 nm of 0.5 nm/cm or lower, i.e., one-fourth of thevalue conventionally allowed in the art.

[0048] As described in the foregoing, in the present invention, studieswere made on various elements influencing the optical characteristics ofquartz glass to determine the optimal range of their concentration.Ideally, the elements above are preferably distributed completelyuniformly over the entire glass body. In practice, however, the idealstate cannot be achieved in view of the transportation of substances andheat during the synthetic process, the heat treatment process, and thediffusion process, and this results in a glass body having somedistribution. The thus established distribution leads to a homogeneousglass body, and the influence thereof is quite large particularly incase the quartz glass is employed in a precision optical equipmentequipped with a F₂ excimer laser.

[0049] Accordingly, in the present invention, particular notice wastaken on the distribution of fluorine concentration, whose slightdifference leads to a fluctuation in refractive index in which basicallyuniformity is required. Thus, in one aspect, the present invention ischaracterized by that the difference between the maximum and the minimumvalues in fluorine concentration is strictly defined to fall within 20mol ppm. In addition to the characteristic described above, in thepresent invention, it was possible to improve the uniformity of thequartz glass as such that the difference between the maximum and theminimum refractive indices in the direction making right angles withrespect to the optical axes (as viewed in the direction along the radiusin case of a lens) should become as low as 2×10⁻⁵ or lower.

[0050] On the other hand, from an industrial viewpoint, an extremelystrict control in conditions is not always preferred because it leads toan increase in production cost. Accordingly, studies were made with anaim to seek for another means for achieving uniformity by canceling outwith a different element even in case the difference between the maximumand the minimum values of fluorine concentration should exceed a valueof 20 mol ppm.

[0051] As a result, the present invention is characterized by that thedifference between the maximum and minimum fluorine concentrations isset within 100 mol ppm, a difference between the maximum and minimumfictive temperatures is set within 50° C., and that the differencebetween the maximum and minimum refraction indices being set to 2×10⁻⁵or lower by properly forming a fluctuation in refractive indices inaccordance with the fictive temperature, in such a manner that thefluctuation in refractive indices attributed to the distribution in theconcentration of fluorine be cancelled—or, by controlling thedistribution in fluorine concentration in such a manner to control theinfluence of the fictive temperature distribution.

[0052] In the present invention, preferably, the fluorine concentrationdistribution is set in such a manner that it forms an approximatelyhemispherical plane with a rotation symmetry having a minimum value atthe axis of rotation symmetry, and the fictive temperature distributionis set in such a manner that it forms an approximately hemisphericalplane with a rotation symmetry having a maximum value at the axis ofrotation symmetry.

[0053] The fluorine concentration distribution in the quartz glass ismainly determined by the density distribution of the soot body and theconcentration of the fluorine-containing gas during the atmospherictreatment as well as that during the vitrification. Thus, by properlycontrolling them, for instance, the concentration at the central portioncan be set higher, or reversely, lower, than that at the outerperipheral portion. It is industrially far easier to allow such adistribution in concentration than achieving a flat concentrationdistribution having completely no fluorine concentration distributionbetween the central portion and the outer peripheral portion. Therefractive index of the quartz glass is influenced by the fluorineconcentration, and it decreases by 1×10⁻⁶ by increasing the fluorineconcentration by 1 mol ppm.

[0054] On the other hand, fictive temperature is a concept introduced byBruckner in the '70s. According to this concept, the structure (physicalproperties) of a glass becomes fixed in case a glass body is cooled fromthe temperature condition during synthesis and heat treatment, andhence, the temperature at which the physical properties are fixed isdenoted as the “fictive temperature”. More specifically, the fictivetemperature influences the density and the refractive indices, and alsoin case of quartz glass, it is known that the density and the refractiveindices change depending on the fictive temperature.

[0055] The distribution in fictive temperature inside the glass bodygenerates in accordance with the difference in cooling rate that isgenerated within the glass body during cooling the quartz glass. Even ifthe original quartz glass body should have some distribution in fictivetemperatures, a flat distribution can be achieved by once holding theglass body at a temperature not lower than the strain point, however, anew distribution can be set in the subsequent cooling process. Since thedistribution in fictive temperature greatly depends on the size of theglass body, shape, and the cooling rate, it is thereby possible torealize a desired distribution by properly selecting the above factors.The refractive index of a synthetic quartz glass is expressed as afunction of the fictive temperature, and roughly, the refractive indexincreases by 1.5×10⁻⁶ by decreasing the fictive temperature by 1° C.

[0056] By combining the two factors influencing the refractive index ofa quartz glass, i.e., the fictive temperature distribution and thefluorine concentration distribution, as described above, thedistribution in refractive index within a quartz glass body can beflattened.

[0057] However, the key point in obtaining a flat distribution inrefractive index by favorably combining the two factors above is thatthe fictive temperature distribution and the fluorine concentrationdistribution can be superposed in such a manner that the distributionform thereof can cancel out each other. More specifically, it isnecessary that the distribution forms are each expressed byapproximately a simple quadratic curve, that these quadratic curves havea common axis of symmetry, that the distribution forms are in a shapecapable of canceling out their influences on the refractive index, and,further, that the distribution width fall within a predetermined rangecapable of canceling out the effect of each other.

[0058] In the fluorine concentration distribution and the fictivetemperature distribution, the distribution should be in a convex or aconcave form constituting a part of a hemispherical plane, provided thatthe apices are set in the axis rotation symmetry. Furthermore, in casethe spherical plane is cut by a flat plane including the axis orrotation symmetry, the crossing lines that are found in the flat planemust be provide a quadratic curve, preferably, an arc. Although thereare cases that the crossing line yield a curve of a higher orderfunction such as those of fourth or sixth order depending on theconditions for doping F and the annealing condition, these distributionforms with higher order functions are not preferred even if they do notyield any inflection points. Still, however, the portion yielding ashape substantially regarded as an arc can be used by cutting off theundesired portions.

[0059] In the description above, “the portion yielding a shapesubstantially regarded as an arc” refers to a case that the differencebetween the distribution curve for the refractive index obtained fromthe curve above and that obtained by a hypothetical arc yields, as shownin FIG. 4, a difference falling within ⅕ of the entire distribution inrefractive index.

[0060] A preferred fluorine concentration distribution and fictivetemperature distribution is described below by making reference to thedrawings.

[0061]FIG. 1 schematically shows the fluorine concentration distributioninside a quartz glass body according to the present invention; in thefigure, the abscissa represents the position, and the ordinaterepresents the concentration. It should be noted that the abscissa andthe ordinate are given in relative units. The ordinate corresponds tothe central axis, and, although the central axis (ordinate) is shown asan axis of symmetry in the figure, the practical distribution is suchwith a rotation symmetry having a rotation axis at the central axis.

[0062]FIG. 2 shows the fictive temperature distribution, and, similar tothe case in FIG. 1, the abscissa represents the position, and theordinate represents the temperature. The ordinate is the same as that ofFIG. 1.

[0063]FIG. 3 shows the distribution in refractive index obtained fromthe fluorine concentration distribution, the same obtained from thefictive temperature distribution, and the refractive index distributionof the quartz glass body obtained by canceling out the former twodistributions. Thus, as described above, it can be understood that aflat distribution in refractive index can be obtained by canceling outthe contribution of the two distribution forms to the distribution inrefractive index.

[0064] Referring to the figures, the ordinate provides a relativeconcentration. However, as described above, the practical controllablerange of distribution should be set in such a manner that the differencebetween the maximum and the minimum fluorine concentration, AF, shouldbe set to 100 mol ppm or lower, and that the difference between themaximum and the minimum fictive temperature, ΔFT, should be set to fallwithin 50° C. If ΔF and ΔFT should fall outside the range defined above,difficulty would be encountered in mutually canceling out thedistribution forms.

[0065] In practice, the fluorine concentration inside a quartz glassbody exhibits an intrinsic distribution at the point of production. Onthe other hand, the fictive temperature distribution can be set tovarious shapes depending on the annealing conditions. Furthermore, thefictive temperature distribution can be homogenized by holding thequartz glass body at a temperature not lower than the strain point for apredetermined duration of time to reset the distribution form acquiredbefore heating. Hence, it is possible to set a more preferred fictivetemperature distribution based on the distribution in refractive indexon observing the resulting distribution form by annealing again thequartz glass body under proper conditions.

[0066] However, it should be noted that, since the quartz glass body isapt to take up impurities during the annealing process from thesurroundings, the atmosphere inside the furnace must be controlled to astate with low impurity concentration. Furthermore, more preferred is totake measures such as applying the annealing after covering the quartzglass body with a vessel made of synthetic quartz glass, etc.

EXAMPLES Example 1

[0067] A high purity SiCl₄ was hydrolyzed in an oxyhydrogen flame, andthe fine silica particles generated thereby were deposited on a rotatingbase body to obtain a white-colored opaque soot body. The soot body thusobtained weighed 2 kg, and yielded a bulk density of 0.25 g/cm³. Theresulting soot body was subjected to a heat treatment inside an electricfurnace at 1000° C. for a duration of 5 hours inside a mixed atmosphereof gaseous nitrogen and gaseous oxygen (mixing ratio by volume:N₂/O₂=80/20) to control the density distribution.

[0068] Subsequently, the soot body was further subjected to a heattreatment for 10 hours under the same temperature condition, but bychanging the atmosphere to that of a mixed gaseous SiF₄ and He (mixingratio by volume: SiF₄/He=3/97), and the resulting product was slowlypassed through a furnace set to a maximum temperature of 1460° C. underan atmosphere of mixed gaseous SiF₄ and He (mixing ratio by volume:SiF₄/He=5/95) at a speed of 1 mm/min to obtain a transparent quartzglass ingot by vitrification.

[0069] The both ends of the thus obtained quartz glass ingot weresupported by left and right chucks of a lathe, and after forming amelting zone by locally heating a part of the ingot with a burner flame,the left and right chucks were rotated at differed rotation times. Byapplying a shear force to the melting zone in this manner, the burnerwas slowly moved while stirring the melting zone to homogenize theentire ingot. To increase the homogeneity, this homogenization operationwas conducted 5 times in total. The homogenized ingot was set inside ahigh purity graphite crucible, and was heated to 1800° C. to shape thequartz glass into a disk 150 mm in diameter and 50 mm in thickness bydeformation utilizing its self-weight.

[0070] The thus obtained shaped body was placed inside a syntheticquartz glass vessel to prevent contamination due to impurities fromoccurring on the body, and the entire vessel was set inside an electricfurnace. Then, after holding it at 1150° C. for 20 hours under theatmospheric condition, the body was gradually cooled to 800° C. at acooling rate of 5° C./hr, at which temperature the current to thefurnace was cut to allow the body to cool to room temperature as it is.Thus was obtained a shaped body for Example 1.

Comparative Example 1

[0071] A quartz glass ingot obtained under the conditions similar tothose described in Example 1 was subjected only once to a homogenizationtreatment by zone melting process, and the entire vessel was set insidea high purity graphite crucible to apply heating and shaping. Then, byapplying a similar operation as that described in Example 1, theresulting product was shaped into a disk 150 mm in diameter and 50 mm inthickness.

[0072] The resulting shaped body was placed inside a synthetic quartzglass vessel to prevent contamination due to impurities from occurringon the body, and the entire vessel was set inside an electric furnace.Then, after holding it at 1150° C. for 20 hours under the atmosphericcondition, the body was gradually cooled to 800° C. at a cooling rate of5° C./hr, at which temperature the current to the furnace was cut toallow the body to cool to room temperature as it is. Thus was obtained ashaped body for Comparative Example 1.

Example 2

[0073] A synthetic quartz glass ingot doped with fluorine was preparedunder the conditions similar to those described in Example 1, and theresulting ingot was subjected to homogenization in a manner similar tothat described in Example 1. The homogenized ingot was set inside a highpurity graphite crucible, and was heated to 1800° C to shape the quartzglass into a disk 150 mm in diameter and 50 mm in thickness bydeformation utilizing its self-weight.

[0074] The thus obtained shaped body was placed inside a syntheticquartz glass vessel to prevent contamination due to impurities fromoccurring on the body, and the entire vessel was set inside an electricfurnace. Then, after holding it at 1150° C. for 20 hours under theatmospheric condition, the body was gradually cooled to 800° C. at acooling rate of 5° C./hr, at which temperature the current to thefurnace was cut to allow the body to cool to room temperature as it is.Thus was obtained a shaped body for Example 2.

Comparative Example 2

[0075] A synthetic quartz glass ingot doped with fluorine was preparedunder the conditions similar to those described in Example 2, and aftersubjecting the resulting ingot to homogenization similarly, theresulting body was shaped into a disk 150 mm in diameter and 50 mm inthickness under the same conditions. The thus obtained shaped body wasplaced inside a synthetic quartz glass vessel, and the entire vessel wasset inside an electric furnace. After holding it at 1150° C. for 20hours under the atmospheric condition, the body was gradually cooled to900° C. at a cooling rate of 20° C./hr, at which temperature the currentto the furnace was cut to allow the body to cool to room temperature asit is. Thus was obtained a shaped body for Comparative Example 2.

Comparative Example 3

[0076] A synthetic quartz glass ingot doped with fluorine was preparedunder the conditions similar to those described in Example 2, and aftersubjecting the resulting ingot to homogenization similarly, theresulting body was shaped into a disk 150 mm in diameter and 50 mm inthickness under the same conditions. The thus obtained shaped body wasplaced inside a synthetic quartz glass vessel, and the entire vessel wasset inside an electric furnace. After holding it at 1150° C. for 20hours under the atmospheric condition, the body was gradually cooled to500° C. at a cooling rate of 1° C./hr, at which temperature the currentto the furnace was cut to allow the body to cool to room temperature asit is. Thus was obtained a shaped body for Comparative Example 3.

Comparative Example 4

[0077] After forming a soot body under the conditions similar to thosedescribed in Example 1, the resulting body was subjected to a heattreatment in an electric furnace at 1000° C. for 5 hours under a mixedatmosphere of gaseous nitrogen and gaseous oxygen (mixing ratio byvolume: N₂/O₂=80/20) to control the density distribution. Subsequently,the resulting body was further subjected to a heat treatment for 10hours under the same temperature condition, but by changing theatmosphere to that of a mixed gaseous SiF₄ and He (mixing ratio byvolume: SiF₄/He=3/97), and the resulting product was slowly passedthrough a furnace set to a maximum temperature of 1460° C. under anatmosphere of mixed gaseous SiF₄ and He (mixing ratio by volume:SiF₄/He=10/90) at a speed of 0.7 mm/min to obtain a transparent quartzglass ingot by vitrification. The thus obtained fluorine-doped syntheticquartz glass ingot was subjected to homogenization in a manner similarto that described in Example 2, shaped, and annealed under the similarconditions to obtain a shaped body for Comparative Example 4.

Example 3

[0078] A synthetic quartz glass ingot doped with fluorine was preparedunder the conditions similar to those described in Example 2, and aftersubjecting the resulting ingot to homogenization similarly, theresulting body was shaped into a disk 150 mm in diameter and 50 mm inthickness under the same conditions. The thus obtained shaped body wasplaced inside a synthetic quartz glass vessel, and the entire vessel wasset inside an electric furnace. After holding it at 1150° C. for 20hours under the atmospheric condition, the body was gradually cooled to960° C. at a cooling rate of 1° C./hr, at which temperature the currentto the furnace was cut to allow the body to cool to room temperature asit is. Thus was obtained a shaped body for Example 3.

Example 4

[0079] The soot body formed by a process similar to that described inExample 1 was subjected to a heat treatment a heat treatment in anelectric furnace at 1000° C. for 5 hours under a mixed atmosphere ofgaseous nitrogen and gaseous oxygen (mixing ratio by volume:N₂/O₂=80/20) to control the density distribution.

[0080] Subsequently, the resulting body was further subjected to a heattreatment for 10 hours under the same temperature condition, but bychanging the atmosphere to that of a mixed gaseous SiF₄ and He (mixingratio by volume: SiF₄/He=10/90), and the resulting product was slowlypassed through a furnace set to a maximum temperature of 1460° C. underan atmosphere of mixed gaseous SiF₄ and He (mixing ratio by volume:SiF₄/He=10/90) at a speed of 1 mm/min to obtain a transparent quartzglass ingot by vitrification.

[0081] The thus obtained quartz glass ingot was set inside a high puritygraphite crucible, and was heated to 1800° C. to shape the quartz glassinto a disk 150 mm in diameter and 50 mm in thickness by deformationutilizing its self-weight.

[0082] The thus obtained shaped body was placed inside a syntheticquartz glass vessel, and the entire vessel was set inside an electricfurnace. Then, after holding it at 1150° C. for 20 hours under theatmospheric condition, the body was gradually cooled to 900° C. at acooling rate of 15° C./hr, at which temperature the current to thefurnace was cut to allow the body to cool to room temperature as it is.Thus was obtained a shaped body for Example 4.

Comparative Example 5

[0083] A synthetic quartz glass ingot doped with fluorine was preparedunder the conditions similar to those described in Example 3, and asynthetic quartz glass shaped body was obtained by subjecting theresulting ingot to homogenization and shaping in a similar manner. Thethus obtained shaped body was placed inside a synthetic quartz glassvessel, and the entire vessel was set inside an electric furnace. Afterholding it at 1150° C. for 20 hours under the atmospheric condition, thebody was gradually cooled to 800° C. at a cooling rate of 2° C./hr, atwhich temperature the current to the furnace was cut to allow the bodyto cool to room temperature as it is. Thus was obtained a shaped bodyfor

Comparative Example 5.

[0084] The shaped bodies obtained in Examples and Comparative Examplesabove were subjected to the measurement of refractive index by using aFizeau interferometer. The birefringence was measured by using a strainmeter under crossed nicols. Then, samples were cut out from each of theshaped bodies to measure the internal transmittance for radiations 157nm and 163 nm, the density of hydrogen molecules, the concentration ofOH groups, the concentration of fluorine, and the distributions thereof.The results are given in Table 1 as shown in FIG. 6.

[0085] The principal physical properties of the quartz glass above,i.e., the fluorine concentration, the density of hydrogen molecules, theOH group concentration, and the fictive temperature, were measured bylaser Raman spectroscopy by a known method. More specifically, thefluorine concentration was measured in accordance with the methoddescribed in J. Material Sci., 28 (1993), pp. 2738 to 2744; the densityof hydrogen molecules was measured according to the method described inJ. Applied Spectroscopy, 46, No. 6 (1987), pp. 632 to 635; the OH groupconcentration was measured according to the method described in Journalof Applied Physics, 37 (1966), p. 3911; and the fictive temperature wasmeasured in accordance with the method described in J. American PhysicalSociety, 28, No. 6 (1983), pp. 3266 to 3271.

[0086] The internal transmittance T as referred in the present inventionsignifies the internal transmittance per a thickness of 10 mm, and canbe calculated by the apparent transmittance D per 10-mm thickness of thespecimen and the theoretical transmittance T₀. More specifically, bytaking a reflection index R and a refractive index n, the values can beobtained in accordance with the equations as follows:

R=(n−1)²/(n+1)²  (1)

T₀=(1−R)²  (2)

T=D/T₀  (3)

[0087] The fluorine concentration in mol ppm as referred in the presentinvention can be obtained by the following conversion equation based onthe concentration expressed by weight ppm.

F mol ppm=0.32×F weight ppm

[0088] The effect of the present invention can be clearly understoodfrom the table given in FIG. 6.

[0089] More specifically, referring to the table, the optical materialsaccording to Examples 1 and 2, which completely satisfy the conditionsof the present invention, exhibited favorable optical characteristics asa synthetic quartz glass optical material for use with F₂ excimer laser.

[0090] On the other hand, the material obtained in Comparative Example1, which yields a ΔF value exceeding 20 mol ppm, it was found that thecorrection was insufficient as a synthetic quartz glass optical materialfor use with F₂ excimer laser, even though a correction for thefluctuation in refractive index due to ΔF was attempted by controllingthe fictive temperature distribution.

[0091] Furthermore, concerning Comparative Example 2, in which ΔF valueexceeded 20 mol ppm and in which a correction using fictive temperaturedistribution was not applied, a further increase was observed to occuron the fluctuation of the refractive index.

[0092] In case of Comparative Example 3, in which the difference inmaximum and minimum fictive temperatures exceeded 50° C., and inComparative Example 4, in which the ΔF value exceeded 50° C., it wasfound that the correction was insufficient as a synthetic quartz glassoptical material for use with F₂ excimer laser, even though a correctionfor the fluctuation in refractive index due to ΔF was attempted bycontrolling the fictive temperature distribution.

[0093] In Example 3, the fluctuation in refractive index was found to belarger than that obtained in Example 2, because the maximum temperatureof the fictive temperature was higher than the preferred range. However,it exhibited optical characteristics sufficient for a synthetic quartzglass optical material for use with F₂ excimer laser.

[0094] In Example 4, the concave and convex shapes in the distributionsof refractive index and fictive temperature were reversed to thoseobtained in Example 2; however, favorable optical characteristics for asynthetic quartz glass optical material for use with F₂ excimer laserwere obtained similar to the case of Example 2. In contrast to this, thesample obtained in Comparative Example 5 showed an increase in thefluctuation of refractive index because no correction was made for thefluctuation in refractive index, ΔF, was made by using the fictivetemperature distribution.

[0095] Furthermore, a sample cut out from the shaped body obtained inExample 2 was irradiated with a F₂ excimer laser radiation toinvestigate the change in transmittance.

[0096] More specifically, the transmittance spectrum obtained on thesample subjected to an irradiation of pulsed F₂ excimer laser radiationsat a pulse energy density of 8 mJ/cm2 for 3.5×105 pulses is shown inFIG. 5.

[0097] Further, several types of quartz glass ingots differing in thefluorine concentration were prepared by changing the concentration ofSiF₄ at 1000° C. under the conditions of preparing the shaped body ofExample 1 and the concentration of SiF₄ during the vitrification processfor obtaining the transparent glass body.

[0098] Then, measurements were made on the fluorine concentration, OHgroup concentration, and the internal transmittance for a radiation 157nm in wavelength before and after irradiating an excimer laserradiation. The measured results are shown in Table 2 given in FIG. 7.

[0099] The effect of the present invention can be clearly understoodfrom the table given in FIG. 7.

[0100] For the shaped body obtained in Example 1, the concentration ofmetallic impurities was measured by using ICP mass analysis and plasmalight emission spectroscopy to obtain results as follows. Transitionmetals Cu 0.2 ppb Ni 0.4 ppb Ti 2.8 ppb Cr 0.3 ppb Fe 6.2 ppb

[0101] Alkali metals Na 8.5 ppb Li 1.7 ppb K 2.6 ppb

[0102] Alkaline earth metals Ca 3.4 ppb Ba 1.2 ppb Mg 1.5 ppb

[0103]FIG. 1 is a graph showing a distribution in fluorine concentrationpreferred in the present invention.

[0104]FIG. 2 is a graph showing a distribution in fictive temperaturepreferred in the present invention.

[0105]FIG. 3 is a graph showing a distribution in refractive indeximplemented by the present invention.

[0106]FIG. 4 is a graph showing the concept of an approximatelyhemispherical distribution.

[0107]FIG. 5 is graph showing a transmittance spectrum obtained afterirradiating a F2 laser radiation to a molded body obtained in Example 2according to the present invention.

[0108]FIG. 6 is a Table showing the physical properties and the like ofthe molded bodies obtained in Examples and Comparative Examplesaccording to the present invention.

[0109]FIG. 7 is a Table showing the OH group concentration and theinternal transmittance for a radiation 157 nm in wavelength obtainedbefore and after irradiating a F₂ excimer laser of a mold ed bodyobtained in Example 1 according to the present invention, in which thefluorine concentration is varied.

1. A synthetic quartz glass optical material for F₂ excimer lasershaving an OH group concentration of 0.5 ppm or lower, a fluorineconcentration of 0.1 to 2 mol %, a hydrogen molecule density of 5×10¹⁶molecules/cm³ or lower, a difference between the maximum and minimumfluorine concentrations within 20 mol ppm, and a difference between themaximum and minimum refraction indices of 2×10⁻⁵ or lower.
 2. Asynthetic quartz glass optical material for F₂ excimer lasers having anOH group concentration of 0.5 ppm or lower, a fluorine concentration of0.1 to 2 mol %, a hydrogen molecule density of 5×10¹⁶ molecules/cm³ orlower, a difference between the maximum and minimum fluorineconcentrations within 100 mol ppm, a difference between the maximum andminimum fictive temperatures within 50° C., and a difference between themaximum and minimum refraction indices being set to 2×10⁻⁵ or lower byrelatively forming a fluctuation in refractive indices in accordancewith the fictive temperature, in such a manner that the fluctuation inrefractive indices attributed to the distribution in the concentrationof fluorine be cancelled.
 3. A synthetic quartz glass optical materialfor F₂ excimer lasers as claimed in claim 2, wherein the maximum valuein the distribution of fictive temperature is 920° C. or lower.
 4. Asynthetic quartz glass optical material for F₂ excimer lasers as claimedin one of claims 1 to 3, wherein the internal transmittance for aradiation 157 nm in wavelength emitted from F₂ excimer lasers is 70% orhigher.
 5. A synthetic quartz glass optical material for F₂ excimerlasers as claimed in one of claims 1 to 3, wherein the internaltransmittance for a radiation 163 nm in wavelength is 90% or higher. 6.A synthetic quartz glass optical material for F₂ excimer lasers asclaimed in one of claims 1 to 3, wherein the drop in transmittance for aradiation 157 nm in wavelength after irradiating thereto 3×10⁶ pulses ofF₂ excimer laser radiation at an energy density per pulse of 10 mJ/cm²is 5% per 10 mm or less.
 7. A synthetic quartz glass optical materialfor F₂ excimer lasers as claimed in one of claims 1 to 3, wherein thebirefringence measured at a wavelength of 633 nm is 0.5 nm/cm or lower.8. A synthetic quartz glass optical member for F₂ excimer lasers formedby using a synthetic quartz glass optical material as described in oneof claims 1 to 7.